Heretofore, certain electroconductive materials such as metals and carbon materials have been used in fields wherein a high electroconductivity is required. On the other hand, in recent years, electroconductive materials have been used in various ways in many fields such as electronics, electrochemistry, energy and transportation equipment. Along with such usage of the electroconductive materials, an electroconductive resin composition, as a kind of the electroconductive material, have played a more important role. As a result, the electroconductive resin compositions have made remarkable progress so as to permit higher performances and higher functions. Particularly, the degree of freedom of the molding workability thereof is expanded due to the combination of the-mentioned electroconductive material and a polymer material, and this is a strong reason why the electroconductive resin compositions have been remarkably developed.
In the field of the electroconductive resin composition, it is important to effectively provide an electroconductivity, substantially without impairing the mechanical characteristic and molding characteristic thereof. For example, Patent Document 1 discloses a method of mixing two or more kinds of polymers which are not completely compatible with each other so as to provide a matrix, so that a filler for imparting an electroconductivity is predominantly distributed in one of the polymers having a larger affinity therewith.
Recent examples of the usage or application for the electroconductive resin composition in which the electroconductivity is required, may include: in addition to the conventional applications, electronic materials such as circuit boards, resistors, laminates, and electrodes; and various members such as heaters, members constituting heat-generating devices, dust-collecting filter elements, PTC (positive temperature coefficient) elements, electronics elements or parts, and elements or parts to be used in the semiconductor industry. In these applications, a high thermal resistance is required together with an electroconductivity.
On the other hand, in view of environmental problems and energy problems, fuel cells have attracted much attention as clean power-generating devices, because they generate electric power by a reverse reaction of electrolysis by using hydrogen and oxygen, and they produce no exhaust material other than water. Also, in the field of the fuel cell, the electroconductive resin compositions have important roles.
The fuel cells can be classified into several kinds, depending on the kind of the electrolyte to be used therefor. Among such fuel cells, solid polymer electrolyte-type fuel cells can work at a low temperature, and therefore they are most useful for automobile or public or civilian uses. This type of fuel cell is constructed by stacking unit cells, each of which comprises, e.g., a polymer electrolyte, a gas diffusion electrode, a catalyst and a separator, and the fuel cell can attain high-output power generation.
In the fuel cell having the structure, the separator for partitioning the unit cells usually has at least one flow channel (or groove) to which a fuel gas (such as hydrogen) and an oxidant gas (such as oxygen) are supplied, and from which the produced water content (steam) is discharged. Therefore, the separators is required to have a high gas impermeability capable of perfectly separating these gases, and is also required to have a high electroconductivity to reduce the internal resistance. Further, the separator is required to be excellent in heat conductivity, durability, strength, etc.
To satisfy these requirements, the separator has been heretofore studied in view of both aspects of metal and carbon materials. Among these materials, metals have a problem in the corrosion resistance thereof and therefore, an attempt has been made to cover the surface thereof with a noble metal or carbon. However, in such a case, a sufficiently high durability cannot be obtained and moreover, the cost for covering the metal is problematic.
On the other hand, a large number of carbon materials have been studied as materials for constituting fuel cell separators, and examples thereof include a molded article obtained by press-molding an expanded graphite sheet, a molded article obtained by impregnating a carbon sintered body with a resin and curing (or hardening) the resin, a vitreous carbon obtained by baking a thermosetting resin, and a molded article obtained by mixing a carbon powder and a resin and molding the resultant mixture.
For example, Patent Document 2 discloses a complicated process such that a binder is added to a carbon powder and mixed under heating, the mixture is CIP (Cold Isostatic Pressing)-molded, baked and graphitized, and the thus obtained isotropic graphite material is impregnated with a thermosetting resin and subjected to a curing treatment, and grooves are engraved therein by cutting.
In addition, attempts have been made for the purpose of enhancing the performances of the separator by improving the composition to be used therefor. For example, Patent Document 3 discloses a separator which is excellent in both of the mechanical and electric characteristics, based on the composite of a carbon powder coated with a resin and a resin having a higher thermal resistance than the coating resin. Patent Document 4 discloses a resin composition comprising a mixture of a low-melting point metal, a metal powder, a thermoplastic plastic and a thermoplastic elastomer.
On the other hand, it is important to lower the contact resistance of a separator, and the technique therefor has been studied in various ways. For example, Patent Document 5 discloses a method of increasing the area rate of carbon powder, by grinding the surface layer of a separator predominantly comprising a resin.
[Patent Document 1] JP-A (Japanese Unexamined Patent Publication) 1-263156
[Patent Document 2] JP-A 8-222241
[Patent Document 3] JP-A 2003-257446
[Patent Document 4] JP-A 2000-348739
[Patent Document 5] JP-A 2003-282084
In the-mentioned various kinds of molded products comprising the conventional electroconductive resin compositions, it is necessary to increase the amount of an electroconductivity-imparting material to be contained in the molded product in order to impart the molded product with a high electroconductivity. On the other hand, in such a case, it is also inevitable to increase the amount of a resin to be contained in the molded product. Accordingly, it is impossible to obtain a sufficiently high electroconductivity.
In addition, because of the large amount of the electroconductivity-imparting material contained in the molded product, the surface of the resultant molded product inevitably has a low smoothness, and a higher hardness, so that the contact resistance of the molded product produced from of an electroconductive resin composition tends to be deteriorated. In addition, when the surface of the molded product is covered with the binder resin, and the contact resistance thereof is deteriorated, it is necessary to adopt a method of grinding the surface the molded product.
Further, when the production process includes a baking step of heating the molded product at a high temperature of 1000-3000° C. for a long period so as to obtain a high electroconductivity, the time required for producing the molded product becomes longer, and further the production steps become complicated, and the production costs problematically become high.